In the area of paper making there is a frequent need for measuring the fiber suspension in order to evaluate different properties of the fibres, the suspension and the fiber web both during forming and after dying.
One important factor is how well the fiber web dewaters during forming. Improving the efficiency of the paper web forming is a major object in all papermaking. In order to be able to determine the performance of a furnish on the paper machine it is necessary to know how fast water drains from that particular furnish and how much water can be expected to be removed by mechanical means such as vacuum.
One such method is described in U.S. Pat. No. 4,613,406 where a suspension sample of a certain volume is led through a screen. A vacuum pump is arranged on the collection container situated after the screen. A pressure recorder on the collection container monitors the pressure value. During build-up of a fiber bed the pressure difference across the fiber bed is measured as a function of time and provides a graph with distinct inflections, which inflections indicate different stages of dewatering of the fiber bed.
The apparatus according to U.S. Pat. No. 4,613,406 is basically a laboratory instrument rather than a device to be used in a more frequent control of an operating paper machine. In that respect an apparatus according to U.S. Pat. No. 4,969,351 has been developed. The apparatus comprises a container for suspension and a sheet mold mechanism comprising a movable table under the container, where the table includes a number of sheet forming locations. Underneath the container and the table is a dropleg. With the apparatus series of samples may be tested in the above mentioned fashion to determine the dewatering aspects of the samples, where it is possible to perform sequential tests with different sheet weight and to obtain data regarding drainage time versus basis weight.
The apparatus exhibits a rather complex constructional design in that the container and the dropleg are arranged movable to and from the table in order to allow the table to be moved so that a sheet forming location may be aligned with the container and the dropleg. This means that there must be sufficient sealing between the parts in order for the suspension not to leak out. As is very common with pulp suspensions, the fibers tend to get stuck and to build up in various places, which may deteriorate the sealing function, and possibly also the total function of the apparatus.
Further, with the apparatuses described above, the produced fiber bed must be removed mechanically afterwards.
Other measurements include moisture measurements on a fiber bed. In those cases an infrared light source, transmitting two narrow bands of infrared radiation, and a detector are utilised in order to establish the transmittances through the paper. The ratio of these transmittances is a function of the water weight per unit area. U.S. Pat. No. 4,823,008 discloses such an apparatus used in the production line of a paper making machine. However, the measuring of the moisture content is not performed in connection with the measuring of dewatering properties, and includes a device movable transversal to the feeding direction of the web with the source on one side of the web and the detector on the other side of the web.
Other values that can be measured on the fiber bed or the web is the test of the kappa number, which refers to the amount of material remaining in the pulp, where the material is often equated with the lignin content of the fibers. One way of detecting the lignin content is to illuminate the fibers with ultraviolet UV-light and to measure the fluorescence of the lignin, thereby obtaining the kappa number. One such device is disclosed in U.S. Pat. No. 5,486,915.
The devices according to the state of the art are rather bulky and complicated in order to fulfil requirements like compact, easy to use and to clean from previous paper beds and that is capable of performing several different tests if required.